Abstract

The aquatic snails Viviparus aterand V. contectus are sympatric in several places in southern Europe; they differ morphologically, karyotypically and in their life-history traits. Previous work has shown weak introgression of allozyme markers associated with high heterospecific mating rates in a natural population at Lazise in Lake Garda, northern Italy. Snails were kept in small artificial ponds with equal numbers of males and females of both species, and rates of assortative mating, species-specific mating propensities, and the relative frequencies of the reciprocal heterospecific mating pairs ( V. ater males× V. contectus females and V. contectus males× V. ater females) were determined. Randomization tests were used to control statistically for problems inherent in many assortative mating experiments. The extent of assortative mating under relative densities of 50 V. ater :1 V. contectus , which is closer to the natural condition at Lazise, was also determined. Heterospecific matings occurred at a rate of 10·9% at relative densities of 1:1, significantly different from random mating. The heterospecific mating rate of V. contectus was much higher (75%) when it was at low density. Viviparus contectusfemales had significantly higher mating propensities than did V. ater females. In contrast to field data, the relative frequencies of the reciprocal heterospecific mating pairs were similar in the experiments, suggesting that factors other than mate choice may be responsible for the higher frequency of V. ater males× V. contectus females in the field. Assortative mating in Viviparuscomprises an insignificant barrier to introgression even in conjunction with the low fitness of hybrids, especially where the relative population densities in the species differ.

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